Iron-Doped Molybdenum Carbide Catalyst with ... - ACS Publications

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Iron-Doped Molybdenum Carbide Catalyst with High Activity and Stability for the Hydrogen Evolution Reaction Cheng Wan and Brian M. Leonard* Chemistry Department, University of Wyoming, Laramie, Wyoming 82071, United States S Supporting Information *

ABSTRACT: Molybdenum-based materials have been widely investigated recently as promising alternatives to platinum for catalyzing the hydrogen evolution reaction (HER). Molybdenum carbide is one of the most studied transition-metal carbides because of its cheap price, high abundance, good conductivity, and catalytic activity. In order to further improve the catalytic activity of molybdenum carbide, some modifications have been applied. In this paper, a wide range of magnetic iron-doped molybdenum carbide (Mo2−xFexC) nanomaterials were synthesized by a unique amine−metal oxide composite method. The amount of iron dopants was controlled by setting different iron/molybdenum ratios in the precursors. Iron-doped molybdenum carbide nanomaterials were investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, energy-dispersive spectroscopy, Raman microscopy, and X-ray photoelectron spectroscopy. Electrocatalytic HER tests were used to demonstrate the catalytic activity upon addition of a second metal into the lattice of molybdenum carbide. Finally, Ni was also doped into the lattice of molybdenum carbide to prove the generality of the synthetic method and tested for catalytic activity.

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INTRODUCTION In order to sustain the future energy demands for mankind, while not damaging the environment, it is necessary to replace fossil fuels with a clean, renewable, and highly efficient energy source. Molecular hydrogen is one of the most promising candidates as a clean fuel source because its only emission is water.1,2 Recently, water electrolysis to generate hydrogen has attracted a lot of attention, which puts the hydrogen evolution reaction (HER) in the spot light.1,3 Platinum (Pt) is still the most efficient electrocatalyst for HER; however, the high cost and limited supply of Pt necessitate finding an alternative cheap and abundant catalyst for HER applications.4 A series of well-known hydrodesulfurization (HDS) catalysts were shown to be excellent catalysts for HER, such as Ni2P,5−7 NiS2,8,9 CoP,10−13 WP,14,15 FeP,16−18 CoSe2,19 MoP,20−23 Mo2C,24−38 and MoS2.39−46 Molybdenum is much more abundant than Pt, and the United States produces the most molybdenum in the world.47 Due to the known high activity of molybdenum-based materials as catalysts for HDS, molybdenum compounds are potential candidates for replacing Pt as electrocatalyts for HER applications.6−8 Molybdenum alloys like Ni−Mo are quite active but less stable in acidic media,48−51 whereas MoP,22,23 MoB,25 Mo2N,52 MoSe2,53 NiMoNx,48 and Co0.6Mo1.4N254 exhibit both excellent activity and stability in acidic media. Molybdenum carbide is the most widely studied carbide due to its similar electronic structure to Pt.55−57 Furthermore, recent studies by Lee et al. showed that β-Mo2C on a carbon nanotube (CNT)−graphene (GR) hybrid support © 2015 American Chemical Society

performs better as an HER catalyst than Mo2N/CNT−GR, and MoS2/CNT−GR, because Mo2C/CNT−GR exhibits the best conductivity and generates relative low hydrogen binding energy.52 According to our previous report, β-Mo2C demonstrates the best HER performance among four phases of molybdenum carbide and other monometallic carbides.27,58 Various morphologies of molybdenum carbide, which includes nanorods, nanospheres, microflowers, etc., have been also synthesized and studied from an amine−metal oxide composite.59 It was demonstrated that porous nanorods of βMo2C works better as an electrocatalyst for HER than bulk βMo2C.38 Besides morphology contribution, an appropriate catalyst support can also increase conductivity and activity of βMo2C.52,60 Adzic et al. designed several systems of β-Mo2C on various carbon supports for HER and found β-Mo2C supported on carbon nanotube−graphene to have the highest activity.26 However, very few reports show modifications of the crystal structure of molybdenum carbide to fine-tune its catalytic activity. Introducing a second metal to form molybdenumbased bimetallic carbides has been regarded as one possible solution, and a Co6Mo6C support was reported to enhance the catalytic activity of Pt for oxygen reduction reaction.61 Trunschke et al. reported the possibility of doping a second metal into the lattice of β-Mo2C (β-Mo2−xVxC, 0 < x < 0.12).62 Received: February 16, 2015 Revised: May 12, 2015 Published: May 13, 2015 4281

DOI: 10.1021/acs.chemmater.5b00621 Chem. Mater. 2015, 27, 4281−4288

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Chemistry of Materials

Transmission electron microscopy (TEM) pictures were captured on an FEI Tecnai G2 F20 200 kV. To prepare TEM samples, 20 mg of carbide was dispersed in 5 mL of ethanol by ultrasonication for 20 min. Energy-dispersive spectroscopy (EDS) maps were gathered on both TEM and SEM. X-ray photoelectron spectroscopy (XPS) data were collected in a Kratos Ultra DLD X-ray Photoemission Spectrometer. C 1s (284.80 eV) was used to align all the XPS peaks. Brunauer−Emmett−Teller (BET) surface area measurements were collected using a Micromeritics ASAP2020 with nitrogen adsorption data at relative pressures from 0.05 to 0.25. Carbon support information was collected by a 532 IM-52 Raman microscope, with a 532 nm laser, built by Snow Range Instruments. Sample Preparations and Tests for Hydrogen Evolution Reaction (HER). About 10 mg of sample (molybdenum carbide, iron carbide, or Pt on carbon) was mixed with 1250 μL of DI water and 250 μL of 5 wt % Nafion in a 5 mL vial. The mixture was then ultrasonicated for 60 min in a 3510 Branson Bath Sonicator. A 3 μL portion of the suspended solution was deposited on a circular glassy carbon electrode (BASi Stationary Voltammetry Electrodes MF-2012, 3 mm OD), giving 0.28 mg/cm2 mass loading of catalyst. The electrodes with catalysts were dried in a drying oven for at least 2 h at 50 °C, and all electrolytes were argon-saturated 0.1 M HClO4 solution (in a 30 mL container). The electrochemical experiments were conducted in three-electrode system that consisted of a working electrode (catalysts on glassy carbon electrode, Ag/AgCl), a NaCl (3 M) reference electrode (BASi Reference Electrodes, MF-2052), and a graphite counter electrode at room temperature. The scan rate for linear sweep voltammetry is 2 mV/s, while the long-term (9 h) stability of pure β-Mo2C and iron-doped β-Mo2C was tested at controlled potentials (−140 and −240 mV).

Furthermore, Abudula et al. claimed that a very small amount of Fe, Ni, or Co in β-Mo2C (∼1.6%) displayed better activity and stability than pure β-Mo2C toward steam reforming of methanol.63 Co-promoted Mo carbides have been studied as a catalyst for hydrogenation of CO,64,65 methane decomposition,66,67 and hydrogen oxidation.68 Chen et al. reported that Co-modified Mo2C improves the activity, selectivity, and durability of the catalyst for the reduction of CO2 to CO because of the existence of CoMoCyOz.66,69 The interaction between Ni and Mo2C was also studied as an enhanced catalyst for hydrogenation reactions and dry reforming of methane.70−73 However, there was limited characterization of those second metals (Fe, Ni, or Co) in the lattice of β-Mo2C. In this paper, we report for the first time that controllable amounts of iron can be doped into molybdenum carbide nanomaterials with the Fe2N structure (0−8 at %), which was synthesized from a unique amine−metal oxide method.59,74 In addition, the HER catalytic activity and stability of pure βMo2C can be enhanced by having Fe doped into the crystal structure. There are two possible reasons for the enhanced HER activity. First, during the synthetic process of β-Mo2C, a graphitic carbon support was formed in the iron-doped systems. Iron is frequently used as a catalyst for the growth of graphitic carbon.75−78 Graphitic carbon supports are believed to play a crucial role in HER catalysts by improving the conductivity and offering more active sites.12,36,39,79−82 Second, the change of lattice and crystal structure causes a broader valence band for molybdenum carbide and more electrons around the Fermi level, which have been shown to have higher activity.27,83 Finally, Ni-doped β-Mo2C was also developed to demonstrate the possibility of other metal dopants in β-Mo2C lattice.

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RESULTS AND DISCUSSION In Figure 1A, β-Mo2C has a hexagonal crystal structure with an ABAB stacking sequence (space group: P63/mmc; JCPDS 00-

EXPERIMENTAL SECTION

Synthesis of Molybdenum Carbide. All of the β-Mo2C were synthesized by mixing 1.00 g (0.81 mmol) of ammonium heptamolybdate ((NH4)6Mo7O24·4H2O) or 1.167 g (5.665 mmol) of sodium molybdate (Na2MoO4) and 2.76 g (22.66 mmol) of 4-Cl-ophenylenediamine in 60 mL of deionized water (Mo:amine = 1:4). All the chemicals were bought from Mallinckrodt, Certified Reagent Chemicals, and Sigma-Aldrich. The pH value of the mixed solutions was adjusted to lower than 3 by adding hydrochloric acid to ensure formation of the amine−oxide hybrid precipitate. For pure β-Mo2C synthesis, the solution was then heated to 50 °C with stirring for at least 2.5 h, and the precipitate was collected by centrifugation at 5500 rpm for 10 min. For Fe-doped molybdenum carbide synthesis, 0.02− 0.1 molar equiv of FeCl3·6H2O (0.01839 g, 0.1134 mmol to 0.0920 g, 0.567 mmol) and the wet precipitate were then mixed in 60 mL of water. The mixture was heated to 50 °C with stirring on a hot plate until all water evaporated from the solution. After the precipitate was dried, the amine−metal oxide composites made from ammonium heptamolybdate were annealed in a tube furnace under argon at 850 °C for 24 h to form both pure β-Mo2C and Fe-doped β-Mo2C. The amine−metal oxide composites made from sodium molybdate were annealed at 1025 °C for 5 h to make pure β-Mo2C. The ramping rate in the tube furnace was controlled at 100 °C/h, and the final products were cooled down to room temperature naturally. The synthesis of Nidoped β-Mo2C uses the same method as Fe-doped molybdenum carbide with NiCl2·6H2O as the second metal source. All metal (Fe or Ni)-doped β-Mo2C’s were treated with excess HCl aqueous solution to remove any free Fe or Ni compounds in the products. Characterization. Metal-doped molybdenum carbide was characterized by X-ray diffraction (XRD) on a Rigaku Smart Lab X-ray diffractmeter, which uses a Cu−Kα source with a wavelength of 1.54060 Å. All calculated data used for this paper came from the PDF2 database. Scanning electron microscopy (SEM) images were collected on an FEI QUANTA 450 with Shottky Field Emitter Gun.

Figure 1. Crystal structure of β-Mo2C: (A) bulk structure, (B) topdown view of the β-Mo2C (010), (C) side view of β-Mo2C (010), (D) top-down view of the β-Mo2C (002), and (E) side view of β-Mo2C (002).

035-0787). In Figure 1B,C, the top-down and the side views of β-Mo2C (010) are exhibited, respectively. Figure 1D,E shows the top-down and the side views of β-Mo2C (002). For β-Mo2C (010), exposed Mo atoms form two layers with mismatched positions. However, Mo atoms in the β-Mo2C (002) surface form only one exposed layer. The corresponding X-ray diffraction peaks for β-Mo2C (010) and (002) are shown in Figure 2B,C to help demonstrate changes in lattice constant that occurs with doping. Figure 2 shows samples with various Fe:Mo ratios (0−0.08) in the precursors, which all form molybdenum carbide (βMo2C) after being annealed at 850 °C for 24 h. In order to 4282


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following the same trends. To obtain accurate lattice constants for Fe-doped Mo2C, XRD peaks were aligned with crystalline SiO 2 as an internal standard (Figure S2, Supporting Information). The absolute values of lattice parameters and unit cell volumes for β-Mo2C are listed in Table 1. Those values were calculated by the software PDXL. Both a/b and c follow similar linear trends where lattice parameters decrease with increasing Fe atomic percentage from 0% to 8%, which implies that smaller iron atoms substituted for molybdenum atoms randomly in the crystal structure. 10% Fe caused the lattice parameters to increase due to the phase separation and production of γ-MoC. It should be noted that the lattice parameter of c and volume for 8% Fe-doped β-Mo2C are smaller than the smallest values (4.727 and 36.77 Å3) found in the database (PDF2) (Figure 3).

Figure 2. XRD patterns of (A) β-Mo2C (JCPDS 00-035-0787) with various Fe:Mo ratios (0:1 to 0.1:1) in the precursors after being annealed at 850 °C for 24 h. The zoom-in regions show (B) β-Mo2C (010) (34−35°) and (C) β-Mo2C (002) and (011) (37−41°). GC (002) stands for graphitic carbon (002) reflection. Samples with Fe dopants have been treated with HCl.

Figure 3. Lattice parameters of a and c for β-Mo2C with distinct Fe atomic percentage.

make sure all excess iron compounds have been removed, the samples (about 100 mg each) in Figure 2A were treated twice with 30 mL of 0.1 M HCl (pH = 1.3).84 Identical XRD patterns were collected before and after HCl treatment, which indicates that either the amounts of free iron compounds are below the detection limits of XRD or most iron is getting into the lattice of β-Mo2C, which cannot be washed out by HCl. To further investigate the change of lattice parameters of β-Mo2C, peak positions of three peaks have been studied individually. In Figure 2B, the (010) peak of pure β-Mo2C sits at the lowest the angle of all the peaks. By increasing the amount of Fe in the precursors, β-Mo2C (010) shifts to the higher angle, which indicates that lattice parameters of a/b in β-Mo2C decrease after doping Fe into the lattice. The trend stops when the Fe:Mo ratio increases beyond 0.1:1, at which point a second phase of molybdenum carbide (γ-MoC) appears, as seen in Figure S1 (Supporting Information). When the Fe:Mo ratio reaches 2:1, γ-MoC becomes the dominated phase. Similar peak shifts were observed for β-Mo2C (002) and (011) in Figure 2C, which suggests that lattice parameter c in β-Mo2C decreases

Both TEM images of pure β-Mo2C (Figure 4A) and Fedoped β-Mo2C with Fe:Mo = 0.08:1 (Figure 4D) show the same particle shape (nanospheres) and similar particle size (30−50 nm in diameter). The d-spacing (2.285 Å, Figure 4B) and SAED are consistent with the (010), (002), and (010) planes of β-Mo2C. In Figure 4E, the carbide particle in the middle is surrounded by graphitic carbon, which has lattice fringes measured as 3.35 Å. Both carbide and graphitic carbon have been confirmed by the XRD patterns in Figure 2A and SAED pattern in Figure 4F. Graphitic carbon support formation is another crucial aspect of improving catalytic properties for the entire system.12,36,39,79−82 Iron and iron-based compounds are widely used for graphitic carbon synthesis.30 Raman spectra were collected to study the process of graphitic carbon formation with different amounts of Fe in the precursors (Figure S3, Supporting Information). D and D′ bands indicate the disordered or defective carbon, while G and G′ bands originate from the well-ordered GC and strong interaction between the layers.85,86 Pure β-Mo2C has large D

Table 1. Values of Lattice Parameters and Unit Cell Volumes for β-Mo2C with Distinct Fe Atomic Percentages Fe:Mo

lattice parameter

precursors

products (EDS)

0:1 0.02:1 0.04:1 0.06:1 0.08:1 0.1:1

N/A 0.0247:1 0.0495:1 0.0551:1 0.0669:1 N/A

a/b (Å) 3.0117 3.0068 3.0025 2.9999 2.9979 2.9991

(8) (7) (5) (4) (7) (4) 4283

c (Å) 4.7375 4.7328 4.7296 4.7246 4.7222 4.7243

(5) (16) (10) (9) (15) (9)

unit cell volume (Å3) 37.21 37.06 36.93 36.82 36.75 36.80 DOI: 10.1021/acs.chemmater.5b00621 Chem. Mater. 2015, 27, 4281−4288

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of Mo6+ (232.55 eV) in pure β-Mo2C. Additionally, Figure S5B (Supporting Information) shows the characteristic peak of Fe3+ located at ∼711.85 eV in the 6% and 8% Fe-doped β-Mo2C, which is also higher than the binding energy of Fe3+ (710.80 eV) in Fe2O3.87,88 This suggests that Fe3+ and Mo6+ with higher binding energies are forming Fe2(MoO4)3 on the surface of iron-doped β-Mo2C rather than MoO3 and Fe2O3. Fe2(MoO4)3 is a very catalytically active material for methanol and hydrocarbon oxidations and is also a potential catalyst for other important chemical reactions like HER.89−95 In order to investigate bulk binding information on β-Mo2C, 10 min Ar+ sputtering was used to completely remove the oxide contaminations. The chemical states for both Fe and Mo (Figures S6 and S7, Supporting Information) for pure β-Mo2C and Fe-doped β-Mo2C showed no significant difference. Mo 3d showed that the oxidation states for both pure β-Mo2C and Fedoped β-Mo2C are primarily Mo0 (228.65 eV).25,27,41,96 The peak position of Fe 2p3/2 (707.80 eV) is within the range between the position of Fe0 2p3/2 (707.0 eV) and Fe2+ 2p3/2 (709.9 eV), which indicates that iron is partially oxidized, associated with the interactions between iron and carbon in the lattice of carbide.97 The binding energy of the Fe 2p3/2 electron in Fe-doped β-Mo2C is comparable to that in Fe3C (∼707.50 eV) and Fe5C2 (∼707.00 eV).98−100 Unexpected and important information has been collected from valence band (VB) studies on Fe-doped β-Mo2C materials, as seen in Figure 5. Transition-

Figure 4. (A) TEM image, (B) HRTEM image, and (C) SAED pattern of pure β-Mo2C. (D) TEM image, (E) HRTEM image, and (F) SAED pattern of Fe-doped β-Mo2C with Fe:Mo = 0.08:1. (G) STEM image, and its corresponding EDS mapping of (H) Mo and (I) Fe for Fe-doped β-Mo2C with Fe:Mo = 0.08:1. Samples with Fe dopants have been treated with HCl.

and D′ peaks and small G and G′ peaks. Once 2% Fe was added into the precursor, large G and G′ peaks were observed, indicating that there is more ordered carbon in the Fe-doped βMo2C. There are similar amounts of graphitic carbon in the 4, 6, and 8% Fe-doped β-Mo2C, which is more than that in 2% Fedoped β-Mo2C. EDS mapping via TEM in Figure 4G−I proves that both Mo and Fe are uniformly distributed and overlapped in the particles. The corresponding EDS mapping of large areas for all four Fe-doped β-Mo2C samples was collected in SEM and is shown in Figure S4 (Supporting Information). A few Fe-rich spots can be seen in the EDS mapping of two samples with higher Fe:Mo ratios (Fe:Mo = 0.06:1 and 0.08:1) before HCl treatment, which indicates that some free Fe compounds existed. However, no Fe-rich spots can be observed in these two samples after HCl treatment, further indicating that the remaining Fe is in the lattice of β-Mo2C. The Fe amounts in the final products collected from EDS are consistent with the amounts in the precursors. EDS analysis gave 2.47, 4.95, 5.51, and 6.69% Fe in the final products synthesized from the precursors with various Fe:Mo ratios of 0.02:1, 0.04:1, 0.06:1, and 0.08:1, respectively. Again, both Mo and Fe are uniformly distributed and overlap in the particles, which further proves that Fe is in the lattice of β-Mo2C. The existence of Fe in the final products was also verified by X-ray photoelectron spectroscopy (XPS) (Figures S5B and S6, Supporting Information). All XPS spectra are aligned by C 1s (284.80 eV) from carbon tape (Figure S5A). The surface of pure/Fe-doped β-Mo2C can be easily contaminated by oxygen to form oxidized molybdenum/iron (Figure S5, Supporting Information). When Fe was doped into β-Mo2C, there is little metallic Mo exposed (Figure S5A,C, Supporting Information). From the high-resolution Mo 3d spectrum (Figure S5C, Supporting Information), the binding energy (233.00 eV) in the samples of 6% or 8% Fe-doped β-Mo2C is higher than that

Figure 5. XPS spectra of valence bands for β-Mo2C with various Fe:Mo ratios (0:1) (black), (0.02:1) (red), (0.04:1) (blue), (0.06:1) (green), and (0.08:1) (purple) after 10 min Ar+ sputtering.

metal carbides are potential catalysts since they have a similar electronic structure to Pt around the Fermi level. To investigate the VBs, Figure 5 compares the VBs of pure β-Mo2C and the other four Fe-doped β-Mo2C. A peak around 9.3 eV arises once a small amount of Fe (Fe:Mo = 0.02:1) was doped into the lattice of β-Mo2C, which causes more electrons around its Fermi level. This peak keeps growing when more Fe (Fe:Mo = 0.04:1, 0.06:1, and 0.08:1) is doped into the lattice. High Fe dopant systems (Fe:Mo = 0.06:1 and 0.08:1) exhibit very similar VBs. Therefore, the peaks at 9.3 eV originate from Fe dopant in β-Mo2C. Broader VBs (more electrons around the Fermi level) are observed for Fe-doped β-Mo2C than pure βMo2C, which makes them a better candidate electrocatalyst for HER application.27,83 4284


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done using Fe-doped β-Mo2C with Fe:Mo = 0.04:1, which is one of the best catalysts shown in Figure 6A. During 60 h, the current density fluctuated between 3.5 and 5 mA due to the size of hydrogen bubbles on the surface of the working electrode. The polarization curves before and after 60 h stability test are also collected in Figure S8B (Supporting Information), and there is no apparent change. In conclusion, Fe-doped β-Mo2C exhibits excellent electrochemical stability at both low and high potentials. As a comparison, two distinct molybdenum sources (ammonium heptamolybdate ((NH4)6Mo7O24) and sodium molybdate (Na2MoO4) were used to synthesize β-Mo2C and study their influence on final products. The XRD patterns of both amine−metal oxide composites show no significant difference except that NaCl peaks are present in the sample that was made from sodium molybdate (Figure S9A, Supporting Information). Figure S9B shows that pure βMo2C can be synthesized from both precursors. However, clean β-Mo2C made from sodium molybdate required annealing at 1025 °C for 5 h. In Figure S10A (Supporting Information), Raman spectra reveal that β-Mo2C made from sodium molybdate has both G and G′ band peaks with higher intensity than β-Mo2C made from ammonium heptamolybdate. Therefore, more graphitic carbon was formed in β-Mo2C made from sodium molybdate, which is consistent with the result from XRD (Figure S9B). HER evaluation tests (Figure S10B) show that both pure β-Mo2C have similar polarization curves, while β-Mo2C with more graphitic carbon has ∼15 mV more positive onset potential. This confirms previous reports that graphitic carbon does increase HER activity. However, both pure βMo2C samples have decreased activity compared to 6% Fedoped β-Mo2C. Figure S11 (Supporting Information) shows that it is also possible to have other metals (e.g., Ni) doped into the lattice of β-Mo2C. Like Fe-doped β-Mo2C, the lattice constants of Nidoped β-Mo2C also decrease comparing with pure β-Mo2C (Figure S11A). Both XPS and EDS data prove that Ni is in the final product after HCl treatment (Figure S11B,D,E,F). However, unlike Fe-doped β-Mo2C, Ni-doped β-Mo2C does not modify the electronic structure of VB of β-Mo2C (Figure S11B). Figure S11G,H shows that Ni-doped β-Mo2C has a larger particle size of the product (200−500 nm in diameter) and less graphitic carbon than Fe-doped β-Mo2C. Therefore, as-synthesized Ni-doped β-Mo2C gives lower HER activity than either pure or Fe-doped β-Mo2C (Figure S11I).

Figure 6. Polarization curves for (A) pure β-Mo2C (black), Fe-doped β-Mo2C with Fe:Mo = 0.02:1 (red), 0.04:1 (blue), 0.06:1 (green), 0.08:1 (purple), Fe3C (gray), and Pt (gray) on carbon tested for HER activity in 0.1 M HClO4 with a scan rate of 2 mV/s and (B) their zoom-in window. Plots of current density vs time at (C) −140 mV were used to analyze the early overpotentials for these five molybdenum carbides and (D) at −240 mV for pure β-Mo2C and Fe-doped β-Mo2C with Fe:Mo = 0.06:1 under a rotating speed of 1000 rpm.

The Fe-doped Mo2C nanomaterials were tested for HER to evaluate the changes seen in the VB. Figure 6 shows the electrocatalytic activity for both pure β-Mo2C and Fe-doped for β-Mo2C using a standard electrochemical configuration in 0.1 M HClO4. The catalyst loading for all samples is ∼0.28 mg/ cm2. Pt on the carbon support exhibits the highest catalytic activity and Fe3C shows the worst. Fe-doped β-Mo2C with Fe:Mo = 0.02:1 shows a significant improvement for HER activity compared to pure β-Mo2C. The other three Fe-doped β-Mo2C (Fe:Mo = 0.04:1, 0.06:1, and 0.08:1) behave almost identically and exhibit even better HER activity than Fe-doped β-Mo2C with Fe:Mo = 0.02:1. For all four Fe-doped β-Mo2C (Figure 6A,B), their onset potentials are much more positive than that of pure β-Mo2C and approach the onset potential of Pt on carbon. To be a good candidate for HER, the stability of a catalyst in acid solution is another important index to measure. In order to test the stability of pure β-Mo2C and four Fe-doped for β-Mo2C, long-term durability tests were run at both low (−140 mV) and high (−240 mV) overpotentials over 9 h. In Figure 6C, pure β-Mo2C shows very little current (−0.07 mA), while the Fe-doped samples show about 2−3 times the current density (−0.15 to −0.25 mA) at this low potential (−140 mV) over 9 h without electrode rotation, which proves that their onset potentials are real and lower than that of pure β-Mo2C. Big hydrogen bubbles were seen on the surface of Fe-doped βMo2C electrocatalysts during the HER tests, which block the surface of catalyst to reduce the current densities. To reduce this impact, the stability test at −240 mV for both pure β-Mo2C and Fe-doped β-Mo2C with Fe:Mo = 0.06:1 were conducted with a rotating speed of 1000 rpm, and shows twice the current density of β-Mo2C. According to Figure 6C,D, there is no obvious current density changes during 9 h at both −140 and −240 mV for Fe-doped β-Mo2C. A long-term durability test (Figure S8A, Supporting Information) at −240 mV was also

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CONCLUSIONS An amine−metal oxide composite method was used to synthesize a new group of Fe-doped β-Mo2C with enhanced activity and stability for hydrogen evolution reaction. Fe dopants in the lattice of β-Mo2C have been confirmed by XRD, EDS, and XPS. XPS studies on Mo 3d for pure β-Mo2C and Fedoped for β-Mo2C show no significant difference. Additionally, oxidation states for Fe in four Fe-doped β-Mo2C are between 0 and 2+ and did not change with Fe amounts. The partially oxidized Fe in the β-Mo2C causes the variation of their VBs, and broader VBs are obtained. HER activity tests reveal that Fedoped β-Mo2C are more active electrocatalysts than pure βMo2C made from multiple molybdenum sources, which is believed due to the broader valence bands of β-Mo2C, more graphitic carbon supports, and highly active Fe2(MoO4)3 species on the surface after Fe was doped. The applications of Fe-doped β-Mo2C are going to be extended in the future. 4285


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This paper also demonstrates that other materials can be doped into the lattice of β-Mo2C, which may lead to enhanced physical or chemical properties for β-Mo2C.

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ASSOCIATED CONTENT

S Supporting Information *

Lattice constant studies by XRD, elemental analysis by EDS, and XPS studies for Fe/Ni-doped β-Mo2C. Carbon supports were analyzed by Raman spectroscopy. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b00621.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +1 3077662807. Tel: +1 3077664137. E-mail: bleonar5@ uwyo.edu (B.M.L.). Present Address

1000 E. University Avenue, Laramie, WY 82071, USA (B.M.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by University of Wyoming Start-up, and the School of Energy Resources. We are grateful for Dr. Erwin Sabio who assisted with the XPS studies.

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REFERENCES

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